This proposal represents an extension of an ongoing research effort to evaluate the response of a rat rhabdomyosarcoma tumor system to high-energy beams of heavy charged particles. The three major aspects of this new proposal are: (1) A continuation of in vitro measurements of RBE and OER throughout the plateau and peak regions of beams having a variety of depth-dose distributions of interest in patient radiotherapy; (2) the initiation of studies to measure the response of tumors to fractionated schedules of mixed high and low LET radiation; and (3) the development of a new carcinoma cell line with the in vivo to in vitro plating characteristic for cell survival assays that will complement present studies using the R-1 rhabdomyosarcoma tumor line.